All the major browsers already locally track the sites you visit so they can maintain your browsing history, turn links purple, etc. What FLoC adds is that it locally computes a low-cardinality anonymized summary of your browsing history, and makes it available to sites.

So in other words FLOC makes the browser spyware. Clearly the browser history is useful for the user. That is why it is there. The browser company has no right to take that history and turn it into an ad profile. It is just plain wrong.